1. field of the Invention
This invention relates to a light source for use in optical sound recording and reproduction apparatus and more particularly to such a light source wherein the light from a light emitting diode is concentrated to a fine line by means of a thin sheet of fluorescent material.
2. Description of the Prior Art
A typical sound recording system includes a microphone, an amplifier, a transducer, and a recording medium. The microphone reacts to sound vibrations to produce a signal having a component that is characteristic of the sound. The amplifier amplifies the signal from the microphone and the transducer changes the amplified signal from one form of energy to another form which is effective to react upon the recording medium to produce a recorded signal.
In some sound motion picture cameras, the sound recording medium is the light sensitive film itself. In the sound recording portion of such a camera, sound is converted by a microphone from acoustical energy to electrical energy, which is amplified and applied to a modulated light source that exposes a sound track on the moving strip of film. This film, when properly developed, carries a photographic record having variations that are representative of the frequency and intensity of the sound which was recorded. The variations representing the sound vibrations can be variations either in the width of the track or in the density of the exposed track. Several approaches have been employed in sound systems to produce a variable-density sound track. One approach has been to use a constant intensity light source which is masked by a variable width slit called a light valve. The light valve consists of a pair of fine metallic ribbons (clamped at their ends under tension) and arranged parallel to one another to form a slit. The light valve is placed in a magnetic field and the current from the audio amplifier is sent through the ribbons causing them to vibrate in proportion to the intensity and frequency of the sound being recorded, thus changing the width of the slit formed by the ribbons. An image of the slit is focused on the moving photographic film by means of appropriate optics to record a variable density sound track.
Another approach for producing a variable density sound track has been to use a flashing lamp, the light output of which is caused to vary in proportion to the intensity and frequency of sound being recorded. The output of the lamp is masked by a slit and an image of the slit is projected onto the film by suitable optics.
To reproduce the sound after the film has been properly developed, the recording procedure is reversed. Light from a constant intensity lamp is focused on a narrow slit and an image of the slit is focused on the sound track of the moving film. This beam passes through the film and is modulated by the sound track in proportion to the intensity and frequency of the original sound. The modulated beam is intercepted by a photoelectric cell which generates an electrical current in proportion to the amount of light incident on its photosensitive surface. The electrical signal thus generated is amplified and used to drive loud speakers.
It is known in the art to use light emitting diodes (LEDs) as light sources to record sound tracks on photographic film and for playing back the recorded sound tracks. Being solid state devices LEDs are inherently reliable, inexpensive, require low power input, and are easily modulated. One of the main problems, however, in adapting LEDs for such applications has been the ability to get a bright enough image to expose the film. The line that is exposed on the film must be extremely fine, on the order of 10 microns wide by 500 microns long and the use of conventional optical means such as masks and lenses, light pipes, and fiber optics to concentrate the output of an LED to a fine line has not resulted in an output intense enough to adequately expose conventional photographic films.
It is well known in the art, that when a light ray traveling in an optically dense medium, e.g. glass, encounters an interface between the optically dense medium and a less optically dense medium, e.g. air, part of the ray will be refracted at the interface and part of the ray will be reflected back from the interface. As the angle of incidence .crclbar. between the ray and the interface increases, a situation is reached wherein the refracted portion of the light ray is parallel to the interface. For angles of incidence larger than this "critical angle" no refracted portion of the ray exists, giving rise to the phenomenon called "total internal reflection." Total internal reflection does not occur when light originates in the less optically dense medium.
In a sheet of optically dense transparent material surrounded by a less optically dense medium, light which is totally internally reflected from one surface of the sheet will encounter the opposite surface at the same critical angle and will therefore continue to be totally internally reflected from one surface of the sheet to the other until the light emerges at an edge of the sheet. This is the well known phenomenon termed "light piping." In such a sheet of optically dense transparent material, most of the light entering the sheet from the sides will pass on through the sheet since it encounters the outgoing surface of the sheet at an angle less than the "critical angle" required for total internal reflection to take place. Consequently, only a small percentage of the light entering a sheet of optically dense material will emerge at the edge due to light piping.
However, when a fluorescent dye is dispersed in the optically dense material, incoming light that may encounter the surface of the optically dense material at an angle less than the "critical angle" is absorbed by the fluorescent dye and is re-emitted in all directions. A substantial percentage of the light which is re-emitted in all directions by the fluorescent dye will be at an angle greater than the "critical angle" required for total internal reflection and will therefore be light-piped to the edges of the sheet.